GIACIATION OF LOWER COPPER RIVER
contain numerous beautiful waterfalls. Hanging valleys of the second order are also present. The hackly surfaces characteristic of glacial plucking are prominent on some of the oversteepened slopes. The roches moutonn6e form is best developed in the hill of naked rock at the mouth of the Tiekel River where the east-west movement of this large glacial tributary has predominated. The upper limit of glaciation must be 3000 to 5000 feet in the northern portion of the canyon. Above this the irregular superglacial surfaces seem to be due more to weathering during the extreme glaciation than at present, for there are no great accumulations of talus below these precipitous cliffs, as would be the case if post-glacial, high-altitudeweatheringhad produced them, rather than super-glacial weathering with a glacier present below in the canyon to transport the weathered material away.
The canyon winds more than in the broader section south of Tasnuna River, so that one sees overlapping spurs from certain points, but the ends of these spurs are sharply truncated by glacial erosion. The cross-section of the gorge is in some places asymetrical, as in the eight miles from Tiekel to Cleave Creek, where the eastern side of the canyon is much more oversteepened than the western side, which still retains some long, slop-
ing, overlapping and only slightly truncated spurs.
FIQ. 70. COPPEB RIVER CANTON. (After U. S. Geological Survey.)
The same section also shows the rock terrace in which the present stream course-is cut in some places, and which, here and there, produces a system of very low, overlapping spurs.
For about fifteen miles south of Wood Canyon Copper River occupies an ill-defined secondary gorge within its glaciated canyon. The gorge is cut in a rock bench or ter-